Porous metal structure



Patented May 15, 1934 PATENT OFFICE POROUS METAL STRUCTURE William G.Calkins and Anthony J. Langhammer,

Detroit, Mich., assignors to Chrysler Corporation,, Detroit, Mich., acorporation of Delaware No Drawing. Application November '1, 1932,Serial No. 641,604

6 Claims.

This invention relates to an improved composition for the manufacture ofporous metal structures and to the method of making such structures.

More specifically, the invention relates to an improved porous bearingcomposition and method of making porous bearings and particularly to theprovision of an improved lubricating agent in compositions of this kindwhich materially 10 improves the character of the final products.

In manufacturing porous metal structures,

such as porous bearings, it is the practice to compress mixtures ofporous metals into briquette form and to then sinter the briquette in areducing atmosphere at a temperature above the fusion temperature of oneof the powdered metal components and below the fusion temperature of theother.

Comparatively high pressures are employed during the briquette formingoperation, and in order to subject all portions of a charge of suchpowdered metal composition to substantially uniform pressure and toprevent excessive wearing and scoring of the dies used in suchoperations, it is desirable to include a lubricant in the composition.

One of the main objects of this invention is to provide a lubricatingagent for this purpose which has a substantially greater lubricatingcapacity no than the agents heretofore employed and which is alsoadapted to serve as a flux during the sintering operation.

Other objects of the invention are to provide an improved method ofmaking porous metal structures; to provide an improved lubricating agentin powdered metal compositions from which such structures are formedwhich decomposes during the sintering operation and from which at leastsome of its elements are liberated so as to produce additional voids inthe structure; to provide a lubricating agent of this character which isnot hygroscopic and which therefore does not become sticky or tend tolump when exposed to a humid atmosphere; and to provide a lubricatingagent which is not sensitive to humidity changes and which has, whileeven in humid atmosphere, the lubricating capacity required to provideuni- "formity of strength and compactness in substantially long and thinwall porous metal structures.

Further objects of this invention are to provide an inorganiclubricating agent for this purpose which is substantially free from sootproducing elements, such as carbon, so as to prevent discoloration ofthe structure during the sintering operation; and to provide aninexpensive lubricating agent of this kind which is particularly adaptedto be distributed uniformly throughout a mixture of powdered metals.

In manufacturing porous metal bearings or other porous metal structuresin accordance with the invention, the powdered metals, such as copperand tin, are thoroughly mixed together with powdered graphite, and alubricating agent, preferably comprising boracic acid, is incorporatedin the mixture. Boron or other derivatives of boron, such as borax,potassium tetra borate, sodium tetra borate, and boron oxides, may beused for this purpose if desired. Boracic acid is, however, particularlydesirable because it decomposes at the sintering temperatures employedwithout liberating soot forming elements or otherwise discoloring thefinal product. During the sintering operation, the boron derivative actsas a flux and causes a substantially integral union of the juxtaposedsurfaces of the particles of metal.

Briquettes having substantially the size, shape, and contour desired inthe final product are then formed by compressing the above compositionbetween suitable dies under a pressure of substantially 40,000 poundsper square inch. The boracic acid or other derivative of boron which isthoroughly distributed throughout the powdered metal composition, servesas a lubricant during the briquetting operation. It reduces thefrictional resistance to relative movement of the powdered metalparticles, resulting in a substantially uniform distribution of thepressure applied during this operation, and it prevents excessive wearand scoring of the surfaces of the dies. The uniform distribution ofpressure throughout the body of the composition produces substantiallyuniform compactness at all portions of the structure and renders itpossible to compress the powdered composition into the form ofsubstantially long thin wall structures, thereby producing a structurehaving substantial mechanical strength.

The briquettes are then placed in a furnace having either a reducing ora non-oxidizing atmosphere and they are sintered at substantially 1500F. This temperature, which is somewhat higher than the melting point ofthe tin and lower than the melting point of copper, causes the tinparticles to rigidly fuse and bond the powdered metal particlestogether.

During the sintering operation the boracic acid is decomposed and wateris liberated in the form of vapor. The liberation of some of theelements of the boracic acid aids in producing voids in the structurewhich add materially to the porosity thereof. In the case of bearings.the resulting porous structures are then quenched in oil so as tosubstantially saturate the pores thereof.

A suitable copper-tin composition may be formed by mixing togethersubstantially 10 parts by weight of powdered tin, 1 parts of powderedgraphite, 1 part of powdered boracic acid, and 87 parts of powderedcopper. Other combinations of powdered metals, such as copper andpowdered iron, may also be used. A suitable copper-iron composition maybe formedby mixing together substantially 15 parts by weight of powderedcopper, 1 parts of powdered graphite, 1 part of boracic acid, and 82parts of powdered iron. The form of powdered iron known as sponge ironhas been found to be particularly suitable for this purpose. Compressionof the copper-iron composition to briquette form may be accomplishedwith pressures of substantially 90,000 pounds per square inch, and thesintering operation may be conducted at substantially 2150 F. in areducing atmosphere.

The boracic acid remains in a substantially dry finely divided stateregardless of the humidity of the atmosphere to which it is subjected.Therefore, all tendency of the lubricant to become sticky or to lumpwhen exposed to highly humid or normal atmosphere is eliminated. Theabsence of carbon and other soot producing elements in the decompositionproducts prevents discoloration of the structure during the sinteringoperation and the fiuxing properties of the lubricating agent result inthe formation of exceptionally strong bonds between the particles of thepowdered metals which materially improve the mechanical strength of thefinal product.

Although but several specific embodiments of the invention are hereinset forth, it is understood that various changes in the steps of themethod and in the composition employed may be made without departingfrom the spirit of our invention, and it is not our intention to limitits scope other than by the terms of the appended claims.

What we claim is:

1. A composition for the manufacture of porous metal bearings includinga mixture of powdered tin, powdered copper, and powdered graphite, and alubricating agent comprising powdered boracic acid.

2. The method of forming an article of manufacture which consists inmixing powdered graphite and powdered copper with a powdered metalselected from the group of metals consisting of tin and iron, thoroughlydistributing boracic acid throughout said mixture, compressing themixture to a briquette form, and sintering the briquette to form a hardporous structure and to simultaneously decompose said boracic acid andvolatilize some'of the elements thereof.

3. The method of forming an article of manufacture which consists inmixing together powdered tin, powdered copper, and powdered graphite,distributing powdered boracic acid throughout said mixture, compressingsaid mixture to briquette form, and sintering the briquette in areducing atmosphere to form a hard porous structure and tosimultaneously drive off some of the elements of said boracic acid andto effect the solution of the oxides of said powdered metals in theremaining boron compound.

4. A composition for the manufacture of porous metal articles includinga mixture of powdered graphite, powdered copper and a powdered metalselected from the group of metals consisting of tin and iron, and alubricating agent disposed throughout said mixture com prising boracicacid in finely comminuted state.

5. A composition for the manufacture of porous metal bearings includinga mixture of powdered copper, powdered iron, and powdered graphite, anda lubricating agent distributed throughout said mixture comprisingboracic acid.

6. The method of forming an article of manufacture which consists inmixing together powdered iron, powdered copper, and powdered graphite,distributing powdered boracic acid throughout said mixture, compressingsaid mixture to briquette form, and sintering the briquetic to form ahard porous structure and to simultaneously drive off some of theelements of said boracic acid.

WILLIAM G. CALKINS. ANTHONY J. LANGHAMMER.

